It is known to polymerize solutions comprising alkenyl aromatic monomers having a diene rubber dissolved therein to form polyblends having a matrix phase of polymers of said monomers having dispersed therein particles of said diene rubber grafted with said monomers.
Mass and mass/suspension processes have been used to prepare such polyblends. U.S. Pat. No. 3,903,202 is one such suitable process for the continuous mass polymerization of such polyblends and it is hereby incorporated by reference.
The morphology of the rubber particles dispersed in the polyblend is critical to the final properties of the polyblend. Generally, the larger the size of said rubber particles, the greater the toughness and the smaller the size, the higher the gloss. Hence, the size of the rubber particles must be controlled to insure the control of the properties of the polyblend. U.S. Pat. No. 3,903,202 discloses that agitation during the early phases of polymerization disperses the dissolved rubber as particles and that higher rates of agitation generally decreases the size of said particles with lower rates of agitation producing larger particles. Controlling agitation rates then provides a means for sizing said rubber particles. It has been found that lower rates of agitation, however, creates problems of heat transfer and inhomogeneity that are difficult to control when operating at the lower agitation rates needed to produce larger particles. This is particularly true in continuous processes wherein large amounts of monomers are being polymerized and heat and homogeneity control are imperative to process and product control.
The art has disclosed other methods for sizing the rubber particles other than by agitation rates. U.S. Pat. No. 3,660,535 discloses a method wherein a partially polymerized solution of rubber and monomers is recycled into a stratifying type polymerizing system having linear flow. The effluent from the first reactor, having a conversion of about 20 to 38%, is recycled as a partially polymerized solution in varying amounts to the feed stream at a location of essentially zero conversion and prior to phase inversion in the first reactor.
This procedure has been found to vary the particle size of the rubber during the inversion of the rubber phase, however, control has been found to be difficult because the recycle stream contains grafted rubber particles that have varying levels of graft varying with levels of conversion in the recycle stream. The grafted rubber particles in the rubber phase has been found to act as a soap or dispersing agent for the rubber particle in the monomer-polymer phase during inversion of the two phases, hence, minor variations in graft or conversion have a profound effect on particle size control.
This is particularly evident as polymerization is carried out in a linear flow stratifying reactor system wherein inversion gradually occurs as the feed stream is progressively polymerized from 0% to 20 to 38% conversion down through the stratified reactor.
U.S. Pat. No. 3,398,214 disclosed a batch mass polymerization process wherein a partially polymerized monomer batch of 30-40 conversion is blended with a partially polymerized rubber-monomer batch of less than 10% conversion and the blend is then completely polymerized by batch polymerization. The process is operable only when the monomer-rubber solution is prepolymerized to not greater than 10% conversion which provides very small rubber particles for high gloss in molded parts. High toughness is only developed when the monomer-rubber solution is polymerized to about 3% or less. Such processes do not provide high efficiency at such low conversions.
U.S. Pat. No. 3,488,743 discloses a process wherein a solution of monomer, low molecular weight rubbers and polymer are batch mass polymerized then finished by batch suspension polymerization.
Such batch process produce polymers having relatively small rubber particles which do not efficiently toughen the polyblends having relatively low elongation at fail. Beyond properties, the batch processes require long conversion cycles to graft, invert and disperse the rubber phase, hence, having high energy requirements.
It is the objective of the present invention to provide an improved continuous mass polymerization process for monomer-rubber solutions that inverts and disperses the dissolved rubber into rubber particles rapidly such that the rubber particle is controlled as to particle size and particle size distribution providing improved physical property control.
It is also the objective of the present invention to provide an improved continuous mass polymerization of monomer-rubber solutions wherein the improvement comprises adding minor amounts of a polymer of said monomers to the monomer-rubber solution such that the dissolved rubber becomes inverted and dispersed rapidly into rubber particles such that the rubber particles are controlled as to particle size and particle size distribution providing improved physical property control.
The sequence of steps is critical in providing the improved process. The first reaction zone is operating under steady state conversion of about 15 to 50% conversion, such that as said monomer-rubber solution enters said first reaction zone the dissolved rubber inverts into dispersed rubber particles. The first reaction zone is operating under back mixed steady state polymerization such that the partically polymerized solution is homogeneous as to temperature, viscosity and chemical composition causing said dissolved rubber to invert and be sized into rubber particles in a continuous and controlled operation.
The improved process of the present invention provides that the monomer-rubber solution has present a sufficient amount of a polymer of said monomers such that said dissolved rubber phase inverts and is dispersed rapidly into rubber particles, said rubber particles having higher levels of said polymer of said monomer as occluded polymer during inversion that controls the size and size distribution of said particles. During the rapid inversion of the dissolved rubber, there is less grafted rubber particles in the rubber phase as a soap to decrease the particle size, hence, larger particles can be formed at the higher agitation rates needed for heat transfer. Here, the larger the concentration of the polymer added to the monom-er rubber solution, the larger the particle providing a means for readily controlling the rubber particle size. As the rubber particles move through the first and second reaction zones the rubber particles develop a grafted rubber phase which stabilizes the rubber particles at the size formed during inversion so that particles size is controlled in the final product.